Cold-rolled steel sheets to be used in automobile parts such as framework parts require a high strength on the order of 980 MPa or more, so as to have satisfactory crash safety and to reduce fuel consumption due to reduction in body weight. Simultaneously with this, the cold-rolled steel sheets require excellent processability (workability) so as to be processed into framework parts having complicated shapes.
High-strength steels largely used in bolts, prestressed concrete wires, line pipes, and other uses, when having a tensile strength of 980 MPa or more, are widely known to suffer from hydrogen embrittlement (e.g., pickling embrittlement, plating brittleness, and delayed fracture) due to the intrusion of hydrogen into the steel. The delayed fracture is a phenomenon in which hydrogen generated in a high-strength steel due to a corrosive environment or atmosphere diffuses to defects such as dislocations, vacancies, and grain boundaries to embrittle the material steel and to thereby cause fracture upon the application of a stress. The delayed fracture has harmful effects on the metal material, resulting in low ductility and/or low toughness. Most of techniques for improving hydrogen-embrittlement resistance are adopted to steels used typically in bolts. For example, Non Patent literature (NPL) 1 describes that a steel, when having a metal structure mainly containing tempered martensite and further containing one or more elements showing resistance to temper softening (e.g., Cr, Mo, and V), effectively has improved delayed-fracture resistance. This technique suppresses fracture by precipitating alloy carbides and utilizing them as hydrogen trapping sites to allow the delayed fracture to shift from intergranular fracture to transgranular fracture (intragranular fracture). These findings are, however, to be adopted to medium-carbon steels but cannot be adopted as intact to thin steel sheets having low carbon contents, which require satisfactory weldability and workability.
Under these circumstances, the present applicants have developed an ultrahigh-strength thin steel sheet having satisfactory hydrogen-embrittlement resistance, which contains carbon (C) in a content of more than 0.25 up to 0.60 percent by mass, with the remainder including iron and inevitable impurities (PTL 1). In this ultrahigh-strength thin steel sheet, the metal structure after stretch forming with an elongation of 3% includes retained austenite in a content of, in terms of area percentage to the entire structure, 1% or more; bainitic ferrite and martensite in a total content of 80% or more; and ferrite and pearlite in a total content of 9% or less, while the average axis ratio (major axis/minor axis) of the retained austenite grains is 5 or more.
The thin steel sheet excels in strength, elongation, and hydrogen-embrittlement resistance. Even the thin steel sheet, however, is difficult to reliably attain a stretch flangeability at a demanded level (at least 70%, desirably 90%), which stretch flangeability has been more and more valued recently. This is because the retained austenite causes fracture to lower the stretch flangeability.